Impacts of urbanization and climate change on water quantity and quality in the Carp River watershed

Pressure on water resources has reached unprecedented levels during the last decades because of climate change, industrialization, and population growth. As a result, vulnerability to inappropriate water availability and/or quality is increasing worldwide. In this paper, a Soil & Water Assessment Tool (SWAT) model of the Carp river watershed located in the city of Ottawa, Ontario was calibrated and validated. The model was then used to evaluate the individual and coupled impacts of urbanization and climate change on water quantity (discharge) and quality (nitrogen and phosphorus loads). While most of the watershed is currently rural, the headwaters will undergo rapid urbanization in the future, and there are concerns about possible negative impacts on water quantity and quality. Seven scenarios were developed to represent various watershed configurations in terms of land use and climate regime. Future climate time series were obtained by statistically downscaling the outputs of nine regional climate models, ran under representative concentration pathways (RCP)4.5 and RCP8.5. The impacts were evaluated at the main outlet and at the outlet of an upstream sub-watershed that would be most affected by urbanization. Results show that climate change and urbanization's impacts vary greatly depending on the spatial scale and geographic location. Globally, the annual average discharge will increase between 6.75 and 9.34% by 2050, while changes in annual average nitrogen and phosphorus loads will vary between -1.20 and 24.84%, and 19.15 and 23.81%, respectively. Local impacts in sub-watersheds undergoing rapid urbanization would be often much larger than watershed-scale impacts.